Acidic gold cyanide electroplating bath and process



United States Patent ACIDIC GOLD CYANIDE ELECTROPLATING BATH AND PROCESS Eldridge K. Camp, Waterbury, George Dietz, Woodbury,

and Newell W. Mitchell, Waterbury, Conn., assignors to American Chemical & Refining Co., Inc., Waterbury, Conn., a corporation of Connecticut No Drawing. Filed Oct. 28, 1963, Ser. No. 319,541

5 Claims. (Cl. 204-46) This application is a continuation-in-part of our prior application Serial No. 165,300, filed January 10, 1962, entitled, Gold Plating Process Employing a Bath Containing Potassium Gold Cyanide and Sodium Acid Diorthophosphate, now abandoned.

This invention relates to the deposition of metals selected from the group consisting of gold, silver, nickel, and copper on conducting surfaces by a galvanic, catalytic or electrolytic process and of the compositions for making electrolytes for such a process.

One object of the present invention is to provide a process and composition of a bath for depositing such metals on objects so as to produce a continuous layer of such metals suitable for further processing such as welding, diffusing, soldering, and having improved etch and wear resistance.

Another object of the present invention is to provide a process and bath composition of the above nature which will produce a bright and relatively hard plated layer of fine grained pure metal of any desired thickness.

A further object is to provide a plating bath of the above nature which will have improved throwing power and plating uniformity.

A further object is to provide a commercial electroplating bath of the above nature which will have unusual stability and long life, which will not substantially lose its effectiveness during periods of nonuse, and which will facilitate maintenance and control under continued or intermittent production conditions.

A further object is to provide an improved commercial process and bath for electroplating a high purity gold, which will withstand high temperatures during the diffusion and welding required for solid state device manufacture.

A further object is to provide a plating process of the above nature in which a sparingly soluble gold cyanide (AuCn) mother of pear precipitate will be formed in the agitated bath after a short period of use, and which will remain in suspension and will not collect substantially on the walls of the plating tank.

A further object is to provide a novel bath and process of the above nature which will be inexpensive to operate, and very efiicient and durable in use.

It has now been found that the foregoing and related advantages can be readily attained by a process in which there is dissolved in water potassium gold cyanide and sodium acid diorthophosphate to produce an electrolyte having a pH of about 0.3 to 3.0. A sparingly soluble suspension of a finely divided gold-containing precipitate is produced in the electrolyte and thereafter a workpiece is immersed in the precipitate-containing electrolyte and electroplated to develop a layer of pure gold thereon. Generally, the potassium gold cyanide comprises about 12.0 to 94.0 grams per liter, and the sodium acid diortho- "ice phosphate comprises about 8.0 to 6000 grams per liter.

Applicants have discovered that it is not necessary to filter this precipitate from the bath, but that this suspension will remain in equilibrium with the metal ions in solution, and will dissolve as needed to replenish the dissolved metal ions in the bath as the plating progresses.

The process and bath herein disclosed is unique in the art of gold plating, as the precipitate (AuCn) which remains in suspension, is not deleterious to the electrolyte, but in fact, is quite helpful to the operation of the bath as the gold is more uniformly deposited into the very fine fissures of the articles :being plated. Moreover, the cathode potential may be held under very close control because the gold content of the electrolyte remains practically constant due to the gradual dissolving of the suspension of AuCn, which maintains the dissolved gold content at an equilibrium during the entire electrodeposition process. In other words, the sparingly soluble suspension of AuCn contributes to bath stability, gold plating rate control, and the leveling of the electrodeposited gold.

We have found that substantially any nonoxidizing inorganic or organic acid may be used to produce the desired low pH value of the bath. Suitable low pH values have been found to be those within the range of from about 0.03 to about 3.0, inclusive.

We have also found that silver, nickel, and copper baths may be used for producing smooth and level deposits utilizing a highly acid metal cyanide bath.

There are a number of outstanding advantages to be gained from a bath of this type. First and foremost: The semi-conductor industry needs plating baths that are pure. This type of bath will fill that need, since the control established by the sparingly soluble suspension of metal cyanide will act as a substitute for polarizers, wetters, complexers and other contaminating type additives usually added to plating baths in general use.

Chance organic matter, dust, and other deleterious impurities are apparently adsorbed by the extensive surfaces of the suspended precipitate before it reaches the cathode, and this occurs without seriously altering the concentration of the metal ions in solution. This phenomenon serves to extend the time between activated carbon treatments for removal of impurities during production conditions. Further, since the precipitate serves as an additive, bright level deposits can be obtained.

The hardness, for instance, of such a bright gold plate can be as high as 1'75 Knoop, compared with about 65 Knoop for the lower hardness conventional gold plate. Obviously, since no other metal is present, the deposit will be or 24 karat gold, in contrast to previous bright plates which are usually alloys of gold.

Moreover, applicants gold plating bath is compatible with present day job plating methods and is very easy to maintain and control over extended periods of time. This close control which is produced as a result of the suspended precipitate, avoids the need for close pH control, or buffering.

It is believed that the low metal concentrations involved cause a film of cathodic hydrogen to act as a polarizer, consequently affording an excellent microthrowing powerinto minor depressions. The lack of additives also serves to enhance the integrity of the metal deposited at high temperatures.

Moreover, leveling can be controlled readily by the plater by suitably varying 'the concentration, agitation and temperature of the bath. The proven stability of this type of bath provides a versatility and durability in production conditions, producing a universalness unheard of in usual type baths.

The electrolyte of the present invention will plate bright gold over a wide range of current densities: thus, Hull tests show two bright ranges, one from zero to 20 amperes per square foot, and another 120 to 150. amperes per square foot.

In practice, a current density of between 10 and 2-0 amperes per square foot for rack gold plating has proven most practicable, as compared to conventional gold plating which is usually done below 5 amperes per square foot. With barrel plating a current density of 3 to 8 amperes per square foot will yield excellent deposits. When the suspension volume is kept low (by reducing the metal cyanide content and increasing the acidity) a lower efficiency plating bath results. This is desirable when extremely close control on thickness of very thin films is required. Another advantage is that a more uniform thickness from piece to piece in a barrel load may be thus obtained.

Another advantage is that the efiiciency drops ofi sufficiently with increased local current density to increase the uniformity of the deposit thickness on oddly shaped articles.

Conversely, when the suspension volume is high, the current etficiency and plating rates will be increased. This .gives the advantage of less time in the plating bath when thicker deposits are needed. An additional advantage comes in barrel plating through the burnishing action of the suspension.

A variety of colors are possible through the addition of the silver, copper, and nickel to the bath, the latter providing a beautiful white gold alloy.

For a one liter gold plating bath, the conducting electrolyte bath may be prepared by dissolving an acid, or a salt and an acid, in water to 750 ml. of volume. A solution of potassium gold cyanide will then be slowly added. It is better to employ a hood during this operation as some HCN may be released.

For aging the bath, it will be brought to 180 F., held at said temperature for an hour, and allowed to cool. When time does not permit such aging, the bath may be force-cooled to plating temperature. Agitation will increase the current efliciency and facilitate plating of extra heavy deposits.

Example 1 Potassium gold cyanide g 12 Sodium acid diorthophosphate g Water -ml 1000 pH=2.0.

Example 2 Gold as potassium gold cyanide g 4 Sodium acid diorthophosphate ig 80 Water ml 1000 pH=l.0.

Example 3 Potassium copper cyanide -g 8 Sodium acid diorthophosphate lg 80 Water ml.. 1000 pH=1.3.

Example 4 Potassium nickel cyanide g 40 Sodium acid diorthophosphate -g 120 Water ml 1000 pH=1.6.

One advantage of the improved baths herein disclosed is that they will operate with a wide percentage range of the potassium cyanide metal salt as well as the amounts of sodium acid diorthophosphate brightener, and the pH employed.

While there have been disclosed in this specification five examples in which the invention may be embodied, it is to be understood that these examples are given for the purpose of illustration only, and that the invention is not limited to these specific examples, but may be modified and embodied in various other equivalent forms, without departing from its spirit. In short, the invention includes all the modifications and embodiments coming within the scope of the following claims.

Having thus fully described the invention, What is claimed as new and for which it is desired to secure Letters Patent is:

1. In the process of electrodepositing bright, hard pure gold on a conductive surface, the steps of dissolving in water about 12.0 grams of potassium gold cyanide, about 45.0 grams of sodium acid diorthophosphate and a neutral 7 salt to increase the conductivity of the electrolyte to form The sparingly soluble suspension precipitate develops slowly, but it can be hastened by heating the bath to boiling, and then allowing it to cool over night. While the efficiency of most prior gold plating baths decreases with use and aging, we have found that the novel bath herein disclosed improves with time, and does not deteriorate during use even when idle for long periods.

The conductivity may be improved somewhat by adding a neutral conducting salt, such as potassium nitrate (KNO to the electrolyte bath. The addition in small amounts of such a salt results in smaller voltage requirements, and such an additive does not appear to diminish the brightness or hardness of the gold plate.

Another advantage in the slowly soluble suspended precipitate bath herein disclosed, is that its activity can be controlled for both the electroless and immersion type of chemical and catalytic plating, and a buffer is unnecessary since the precipitate solubility monitors such activity through its close control of the dissolved metal ion concentration.

The following are five examples of plating baths which will yield mirror bright deposits of metal having a greatly superior hardness, as compared with conventional baths.

'In the following examples, the amounts given are in grams and milliliters, and the plating is preferably done in each instance at room temperature with agitation of the bath, using insoluble anodes.

a liter of electrolyte having a pH value of about 2.0; producing in said electrolyte a sparingly soluble suspension of a finely divided gold-containing precipitate; and elec troplating a layer of pure gold on the'surface of a Workpiece immersed in said precipitate-containing electrolyte at room temperature and at a current density of 10 to 20 amperes per square foot.

2. In the process of electrodepositing bright, hard pure old on a conductive surface, the steps comprising dissolving in water potassium gold cyanide and sodium acid diorthophosphate to produce an electrolyte having a pH of about 0.03 to 3.0; producing in said electrolyte a sparingly soluble suspension of a finely divided gold-containing precipitate; and electroplating a layer of pure gold on the surface of a workpiece immersed in said precipitatecontaining electrolyte.

3. The process of claim 2 wherein said potassium gold cyanide comprises about 12.0 to 94.0 grams .per liter and said sodium acid diorthophosphate comprises about 8.0 to 600.0 grams per liter.

4. The process of claim 3 wherein said electroplating is conducted at about a current density of 10 to 20 amperes per square foot and at room temperature.

5. An aqueous bath for use in the plating of gold on a conductive surface consisting essentially of about 12.0 to 94.0 grams per liter of potassiumgold cyanide and about 8.0 to 600.0 grams per liter of sodiumacid diorthophosphate, the electrolyte formed thereby having a pH of less than about 3.0 and containing a sparingly soluble suspension of a finely divided gold-containing precipitate which 5 6 will dissolve slowly in said bath as needed to replenish 3,104,212 9/1963 Rinker et a1. 20'446 the gold ion in solution in said bath during operation. 3,149,057 9/1964 Parker et a1. 20446 References Cited by the Examiner OTHER REFERENCES UNITED STATES PATENTS 5 Langbein, George et al.: Electro-Deposition of 2,654,702 10/ 1953 DeLong 2104-52 X Metals, pages 322-323, 1924.

2,812,299 11/1957 Volk 20446- X 2,905,601 9/1959 Rinker et a1. 20 4-46 X JOHN H MACK, Primary Examiner 2,967, 1 35 1/196 1 Ostrow et a1. 204-46 X 2,978,390 4/1961 Atwater et a1. 20446 10 KAPLAN, Assistant Examiner- 

2. IN THE PROCESS OF ELECTRODEPOSITING BRIGHT, HARD PURE GOLD ON A CONDUCTIVE SURFACE, THE STEPS COMPRISING DISSOLVING IN WATER POTASSIUM GOLD CYANIDE AND SODIUM ACID DIORTHOPHOSPHATE TO PRODUCE AN ELECTROLYTE HAVING A PH OF ABOUT 0.03 TO 3.0; PRODUCING IN SAID ELECTROLYTE A SPARINGLY SOLUBLE SUSPENSION OF A FINELY DIVIDED GOLD-CONTAINING PRECIPITATE; AND ELECTROPLATING A LAYER OF PURE GOLD ON THE SURFACE OF A WORKPIECE IMMERSED IN SAID PRECIPITATECONTAINING ELECTROLYTE. 